A radiation therapy treatment apparatus is typically used for treatment of cancer in the human body. The technology has been developed towards an increased precision in where the radiation dose is delivered. The goal is to be able to treat only the tumor cells and not the surrounding healthy tissue, in order to limit the negative effects on the healthy tissue subjected to radiation. Before the treatment is actually carried out, a treatment plan is set up and tested on a phantom representing the patient. One desired part of that process is to keep track on how the radiation generator is positioned during the treatment and to monitor the dose given in the different positions. This has been provided by ScandiDos AB in their product called Delta4 treatment system, which comprises a radiation detector mounted at the collimator for monitoring the dose that is actually provided to the patient and for comparison with the conditions measured in a pre-treatment operation by means of a phantom. For an accurate monitoring, information about the position of the radiation detector is required. In the Delta4 treatment system the gantry angle is obtained by means of an angle sensor mounted on the gantry. It would be desired to have the full position of the radiation detector, i.e. the gantry angle as well as the collimator angle from a single position detector.
A prior art device for pretreatment using a phantom, which device comprises a radiation detector to measure radiation levels, is disclosed in WO 2011/098891. In this prior art document it is briefly mentioned that the device can include measurement devices, such as inclinometers, gyroscopes, or gyrometers, which are integrated in the phantom or connected to it, and that such a measurement device can be used for determining spatial orientation of the gantry, the collimator, the table or a 2D radiation detector. However, there are no instructions of how the gantry angle and collimator angle are supposed to be measured by means of the measurement device.